The present invention relates to thrust bearings and a method of fabricating thrust bearings.
Various thrust bearings are employed to facilitate sliding motion of a rotating body. A usual axial piston pump includes a cylinder block within which a plurality of piston shoes are arranged. A swash plate as a thrust bearing is disposed in the cylinder block and operatively associated with the piston shoes. When the cylinder block is rotated, the swash plate causes the piston shoes to reciprocate within the cylinder block. This movement of the piston shoes forces oil out of a cylinder so as to produce a substantial amount of force. To withstand high loads, the swash plate is resistant to wear and exhibits a high degree of lubricity.
Typically, a thrust bearing is composed of a steel plate as a backing plate and a bearing layer joined to one side of the backing plate. The bearing layer is made, for example, of a composite lead/bronze metal. The use of the steel plate presents high mechanical strength and reduces the fabrication cost of the thrust bearing. One example of fabricating a thrust bearing is to introduce a molten alloy into a mold wherein an elongated steel plate is placed. The molded article is passed between a set of rolls to reduce the thickness of the molded article to a predetermined level. A punching press is used to cut the molded article into a plurality of rings as thrust bearings. This process is, however, problematic since oxides and slag are formed during introduction of the molten alloy. Formation of oxides and slag retards sliding motion of a rotating body. It is, also, cumbersome to reduce the thickness of the molded article to a suitable level. Another example of fabricating a thrust bearing is to spray droplets of atomized metal wires against a backing plate. This process, known as a thermal spray process, is problematic since it is cumbersome to prepare metal wires. It is, also, time-consuming to coat the backing plate with the metal alloy. A further example of fabricating a thrust bearing is to press a web of bearing alloy and a web of steel plate against one another. The thrust bearing thus fabricated dose not withstand high loads since only a low degree of bonding strength between the bearing alloy and the steel plate is achieved.
Today, a sintering process is most widely used to fabricate a thrust bearing. In this process, alloy powder is uniformly distributed on one side of a web of steel plate and then, heated, say, to a temperature of 800xc2x0 C. in a reducing atmosphere. During heating, the alloy powder is sintered to form a bearing layer which is, in turn, joined to the steel plate. The bearing layer is porous and thus, susceptible to damage upon exertion of high loads. To fill bores in the bearing layer, the sintered article is passed between a set of identical rolls. Thereafter, the roll-formed article is again heated to improve its integrity or bonding strength. A punching press is employed to cut the semifinished article into a shape suitable for use as a thrust bearing.
A disadvantage with the thrust bearing thus fabricated is that accurate adjustment of the thickness of the thrust bearing is troublesome. This is because the relatively soft bearing layer is more extended than the relatively hard backing plate when the sintered article is passed between the rolls. Another disadvantage is that the thrust bearing does not withstand high loads. During the roll-forming process, the bearing layer is pressed against the backing plate along its length. As a result, the bearing layer has a straight, parallel crystalline arrangement. Where a rotating body is slid in the direction of circumference of the thrust bearing, the rotating body is moved in a direction transverse to the crystalline direction of the bearing layer. This retards smooth sliding motion of the rotating body and causes separation or seizing of the bearing layer.
Accordingly, it is an object of the present invention to provide a durable thrust bearing and a method of fabricating a thrust bearing, which can withstand high loads and allows for accurate adjustment of the thickness of the thrust bearing.
According to one aspect of the present invention, there is provided a thrust bearing comprising a backing plate made of metal and a bearing layer made of metal alloy and joined to one side of the backing plate by a sintering process. The bearing layer is roll-formed in a spiral fashion. As a result, the bearing layer has a spiral crystalline arrangement. This arrangement facilitates sliding motion of a rotating body and effectively prevents mechanical fatigue or seizing of the bearing layer. A pair of bearing layers may be joined to opposite sides of the backing plate. The bearing layer is preferably made of a lead-free alloy composed, for example, of copper, tin and silver.
According to another aspect of the present invention, there is provided a method of fabricating a thrust bearing which comprises the steps of preparing an elongated metal plate, cutting the metal plate into a predetermined shape so as to provide a backing plate, evenly distributing alloy powder on one side of the backing plate, heating the backing plate and the alloy powder to a temperature below the melting point of the alloy powder in a reducing atmosphere whereby the alloy powder is bonded to form a bearing layer, and the bearing layer is also joined to the backing plate to provide a sintered article, rotating the sintered article about its central axis, roll-forming the sintered article in a spiral fashion by pressing the bearing layer against the backing plate, and heating the roll-formed article to a temperature below the melting point of the alloy powder in a reducing atmosphere.
During the roll-forming step, the bearing layer is pressed against the backing plate, as opposed to the prior art process wherein the bearing layer and the backing plate are pressed against one another between a pair of identical rolls. This step facilitates adjustment of the thickness of the thrust bearing.